A rear wheel steering (RWS) is constituted by a motor, a decelerator, and a controller, and steers a rear wheel of a vehicle according to a driving situation of the vehicle to provide drivability and steerability.
In particular, the rear wheel steering (RWS) steers a rear wheel and a front wheel in opposite directions to each other in low-speed driving and steers the rear wheel in the same direction as the front wheel in high-speed driving, and as a result, a turning radius is reduced at a low speed and operating stability is improved.
FIG. 1 conceptually illustrates that the rear wheel operates in the rear wheel steering (RWS) and FIG. 2 illustrates a change of a gain depending on a vehicle speed in an example of controlling a steering angle of the rear wheel steering depending on the vehicle speed.
A most basic control method of the rear wheel steering is a method that controls phases of a front steering angle and a rear steering angle according to the vehicle speed as illustrated in FIG. 1.
That is, as illustrated in FIG. 1, a basic concept is that the rear wheel is steered in an opposite direction to the front wheel at the low speed and the rear wheel is steered in the same direction as the front wheel at the high speed.
In this regard, the size of the rear wheel steering angle is determined by applying Equation X given below and a gain used in Equation X varies as illustrated in FIG. 2 according to the vehicle speed. The gain in Equation X can be calculated from the assumption that a steady state side slip of the vehicle is 0.Rear wheel steering angle=gain×front wheel steering angle  [Equation X]
However, the rear wheel steering angle control method in the related art as a feed-forward method that does not feedback the driving behavior signal but controls the driving behavior signal only by the front wheel steering may have a problem in that robustness deteriorates against a change in control environment such as a road surface condition and a vehicle weight.
Yet another control method as a method that feeds back yaw rate and uses the yaw rate in controlling the rear wheel steering angle controls the rear wheel steering angle by Equation Y given below.Rear wheel steering angle=(gain 1×front wheel steering angle)+(gain 2×yaw rate)  [Equation Y]
Gain 1 and gain 2 used herein may be calculated from the assumption that the side slip is 0 in both a steady state and a transient state.
The control method feeds back a yaw rate signal to minimize the side slip and can acquire a more excellent side slip reduction effect than the method according to Equation X through the feedback of the yaw rate signal, but has a limit in controlling the yaw rate itself except for the side slip.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.